In motor vehicles, vibration dampers serve to ensure that the vibrations of the chassis quickly decay as sprung masses, in order to ensure the driving stability and to achieve a desired driving comfort. For this, usually hydraulic vibration dampers are utilized, in which an axially slidable piston is guided in a cylinder filled with oil. On the piston there is arranged a piston rod that is led upwardly out of the cylinder in a sealed manner and secured on the vehicle chassis. On the other hand, the bottom end of the cylinder is preferably mounted on a wheel or axle part. Thereby, the piston divides the cylinder into an upper extension chamber and a lower retraction chamber, which are connected with one another via at least one throttle valve. In order to compensate for a retracting or submerging piston rod, the retraction chamber is additionally still connected with a gas pressure chamber, in which a pre-pressure of approximately 20 to 30 bar is introduced, in order to avoid a cavitation. In order to regulate or control the damping forces, the throttle valves can be embodied to be electromagnetically adjustable, in order to be able to adapt the driving behavior of the motor vehicle to a prescribed vibration damping and/or a desired driving comfort. However, such throttle valves are relatively slow in their regulating behavior, so that often rapid vibration variations cannot be damped quickly enough.
From the DE 10 2007 026 378 A1, a vibration damper for motor vehicles is known, which is embodied with an electrorheological throttle valve and thereby enables a very rapid vibration regulation. In that regard, a spaced-apart electrode tube is arranged around the inner cylinder tube and together with the inner cylinder tube forms a throttle gap. In that regard, the throttle gap is connected with both the retraction as well as the extension chamber, wherein the viscosity of the electrorheological fluid flowing through the throttle gap is controllable via a high voltage between the electrode tube and the inner cylinder tube. However, this vibration damper, for the volume compensation of the piston rod, comprises an additional pneumatic pressure medium cylinder with a further piston, which is axially connected with the hydraulic cylinder as a so-called one-tube damper. Thereby, the vibration damper becomes not inconsiderably longer, which can lead to installation problems.
A further electrorheological vibration damper is known from the EP 0 261 427 B1, which is embodied as a so-called two-tube damper. In that regard, the shock absorber or shock damper consists of an inner cylinder tube, around which three further spaced-apart cylinder tubes are arranged coaxially as electrodes, which form three valve gaps around the inner cylinder tube. A further spaced-apart outer tube is additionally arranged coaxially to the electrode tubes, and is sealed relative to the extension chamber and is connected with the retraction chamber via a non-return valve that opens in connection with an extending piston, and forms a pneumatic gas pressure chamber. Because the throttle gaps are connected at the bottom with the gas pressure chamber and at the top with the extension chamber, and this represents a system of the communicating tubes or pipes, especially after longer service times this can lead to a portion of the gaseous pressure medium getting from the gas pressure chamber through the throttle gaps into the extension chamber and disadvantageously influencing the vibration damping.
Moreover, the two-tube damper also effectuates a relatively large outer diameter due to the three radially adjacently arranged throttle gaps, so that such an installation space, e.g. within helical coil springs, often is not available.